In general terms, medical valving devices often act as a sealed port that may be repeatedly accessed to non-invasively inject fluid into (or withdraw fluid from) a patient's vasculature. Consequently, a medical valve permits the patient's vasculature to be freely accessed without requiring such patient's skin be repeatedly pierced by a needle.
To those ends, as a preliminary step, medical personnel insert a syringe into a medical valve that is appropriately secured to a patient. For example, the valve may be coupled to a catheter having an opposite end secured within the patient's vain. Once inserted, fluid may be freely injected into or withdrawn from the patient. Problems arise, however, when the syringe is withdrawn from the valve. Specifically, a back pressure (i.e., a proximally directed pressure) produced by the withdrawing syringe undesirably can cause blood to be drawn proximally into the valve (e.g., via an attached catheter). In addition to coagulating and impeding the mechanical operation of the valve, blood in the valve (or in the catheter) also compromises its sterility.